Many natural and managed
ecosystems may change abruptly or non-linearly during the 21st century.
The greater the magnitude and rate of the change, the greater the risk of
adverse impacts.

4.18

Changes in climate could
increase the risk of abrupt and non-linear changes in many ecosystems, which
would affect their biodiversity, productivity, and function. For
example, sustained increases in water temperatures of as little as 1°C,
alone or in combination with any of several stresses (e.g., excessive pollution
and siltation), can lead to corals ejecting their algae (coral bleaching;
see Figure 4-3 and Question
2), the eventual death of the corals, and a possible loss of biodiversity.
Climate change will also shift suitable habitats for many terrestrial and
marine organisms polewards or terrestrial ones to higher altitudes in mountainous
areas. Increased disturbances along with the shift in habitats and the more
restrictive conditions needed for establishment of species could lead to
abrupt and rapid breakdown of terrestrial and marine ecosystems, which could
result in new plant and animal assemblages that are less diverse, that include
more "weedy" species, and that increase risk of extinctions (see
Question 3).

Ecological systems have
many interacting non-linear processes and are thus subject to abrupt changes
and threshold effects arising from relatively small changes in driving
variables, such as climate. For example:

Temperature increase beyond a threshold, which varies by crop and
variety, can affect key development stages of some crops and result
in severe losses in crop yields. Examples of key development stages
and their critical thresholds include spikelet sterility in rice (e.g.,
temperatures greater than 35°C for more than 1 hour during the flowering
and pollination process greatly reduce flower formation and eventually
grain production), loss of pollen viability in maize (>35°C),
reversal of cold-hardening in wheat (>30°C for more than 8 hours),
and reduced formation of tubers and tuber bulking in potatoes (>20°C).
Yield losses in these crops can be severe if temperatures exceed critical
limits for even short periods.

Mangroves occupy a transition zone between sea and land that is set
by a balance between the erosional processes from the sea and siltation
processes from land. The erosional processes from the sea might be expected
to increase with sea-level rise, and the siltation processes through
climate change and other human activities (e.g., coastal development).
Thus, the impact on the mangrove forests will be determined by the balance
between these two processes, which will determine whether mangrove systems
migrate landward or seaward.

Large-scale changes in vegetation
cover could affect regional climate. Changes in land surface characteristics,
such as those created by land cover, can modify energy, water, and gas fluxes
and affect atmospheric composition creating changes in local/regional climate
and thus changing the disturbance regime (e.g., in the Arctic). In areas
without surface water (typically semi-arid or arid), evapotranspiration
and albedo affect the local hydrologic cycle, thus a reduction in vegetative
cover could lead to reduced precipitation at the local/regional scale and
change the frequency and persistence of droughts.

Figure 4-3: The diversity of corals could be affected
with the branching corals (e.g., staghorn coral) decreasing or becoming
locally extinct as they tend to be more severely affected by increases in
sea surface temperatures, and the massive corals (e.g., brain corals) increasing.